Timing signals which make periodic transitions between low and high voltage levels are used in digital electronic systems to control system operation. The signal transitions (or edges) define successive instants at which activities or operations can occur in a digital system.
The timing signals in a digital system are typically produced with the assistance of an oscillator, such as a crystal oscillator, that supplies clock pulses. The power consumption of a system that uses an oscillator to produce clock pulses increases with the oscillation frequency. Although crystal oscillators can reach frequencies of several hundred megahertz, a clock oscillator operating at such a frequency normally cannot be utilized with most state-of-the-art integrated circuits because the circuit power consumption would be too great. Excessive heat would be generated. In addition, a high-frequency oscillator introduces significant electromagnetic interference which is difficult to deal with and reduces system reliability.
Delay lines that delay signal transitions to create desired time intervals are also used to produce timing signals. Unfortunately, precision delay lines typically include inductors which cannot readily be incorporated into integrated circuits. It would be quite desirable to have a low-power technique for generating finely controllable timing signals having sharp transitions.
Hoffman, U.S. Pat. No. 5,073,730, disclose a circuit for controlling peak transition current in data buses of VLSI chips. Hoffman's circuit includes a ring oscillator formed with multiple high speed inverters arranged in a loop to produce a set of control signals with short time delay between them. The control signals are directly used to sequence the switching of the bus drivers so that the transients are spread over a time interval which is relatively short compared to the bus transfer cycle but allows time for some driver transients to end before others begin. In his circuit, Hoffman utilizes the control signals in producing further signals. However, each of the further signals is generated from only one of the control signals and therefore does not have rising and/or falling edges that correspond to transitions of two or more of the control signals.